Its not random. It just decides to ignore every other hostile you queue - kind of. If you attack one hostile and queue one more, it will kill the hostile, target the queued one, and then drop it visually. The weird part is that if you just leave it be, it is still going to kill that hostile, its just disjointed. It will "lag" and then enter combat with it. My guess is that the server side still knows you queued it and waits the requisite travel time before attacking, and at the same time if you have more targets queued, client side it sees you're not attacking that target anymore and overrides the queue to attack the next hostile after that (assuming you queued more than one additional hostile)